I you want to take things a fair way with mods then I'd suggest 3in dump and whatever size back section you can afford/tolerate noise wise. Only drama with that avenue is that it is a size that isn't retailed. It'll have to be custom made, but you can split the cost by doing a custom dump/cat and flanging it for a retail 2.5in cat back i you wanted.
If you don't think you'll go nuts then 2.5in will probably be fine (just atleast try to get 200cell or less cat so you can make the most of it).
Both will need either a phase/stage 2 tune to tick off the cat eficiency code in the software that any dump pipe will likely generate OR you can run a second O2 sensor spacer to mechanically trick the ECU into thinking that flows in/around the cat are normal. Not sure how this would perform on a stock tune (bit strange apparently) but it goes like a demon on my phase 1 custom code with a few tricks.
Oh and in NZ you may or may not have to run a cat. That makes the custom dump cheaper and the milltek 2.5in substantially so.
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